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How many of the nine errors in machining do you know?

Machining error is the degree to which the actual geometric parameters (geometry, geometry and mutual position) of the part after machining deviate from the ideal geometric parameters.


The degree of compliance between the actual and ideal geometrical parameters of the part after machining is the machining accuracy. The smaller the machining error and the higher the degree of compliance, the higher the machining accuracy.


Machining accuracy and machining error are two ways of referring to a problem. Therefore, the size of the machining error reflects the accuracy of the machining. The main causes of machining errors are.


1. The manufacturing error of machine tools


The main manufacturing errors of the machine include spindle rotation errors, guideway errors and drive chain errors.


The spindle rotation error is the actual rotation axis of the spindle momentarily relative to its average rotation axis change, which will directly affect the accuracy of the workpiece being machined. The main reason for the spindle rotation error is the coaxiality error of the spindle, the bearing itself, the bearing between the coaxiality error, the spindle winding degree, etc. Guides are the benchmark for determining the relative position of machine components on the machine and the benchmark for machine movement.


The manufacturing error of the guide itself, the uneven wear of the guide and the quality of the installation are important factors that cause the guide error. Drive chain error is the error of the relative motion between the drive elements at the beginning and the end of the drive chain. It is caused by manufacturing and assembly errors in the various components of the drive chain, as well as by wear and tear during use.


2. Geometric error of the tool


Wear and tear is inevitable with any tool during the cutting process, resulting in changes in the size and shape of the workpiece. The effect of tool geometry error on machining error varies with the type of tool: the manufacturing error of the tool will directly affect the machining accuracy of the workpiece when using fixed-size tools, while for general tools (such as turning tools), the manufacturing error has no direct effect on machining error.


3. Geometric error of the fixture


The role of the fixture is to give the workpiece the correct position for the equivalent of the tool and the machine, so the geometrical error of the fixture has a significant impact on machining errors (especially position errors).


4. Location error


Positioning errors mainly include baseline non-reconciling errors and positioning submanufacturing inaccuracy errors. When a workpiece is machined on a machine, a number of geometrical elements of the workpiece must be selected as the positioning reference for machining, and if the selected positioning reference does not overlap with the design reference (the reference used to determine the size and position of a surface on a part drawing), the reference does not overlap error will result.


The workpiece positioning surface and fixture positioning element together form the positioning vice, due to the inaccurate manufacturing of the positioning vice and the maximum amount of workpiece position change caused by the coordination gap between the positioning vice, called the inaccurate manufacturing of the positioning vice error. Inaccurate positioning sub-manufacturing errors occur only when the adjustment method is used and not when the test-cut method is used.


5. The error caused by the force deformation of the process system


Workpiece stiffness: If the workpiece stiffness in the process system is relatively low relative to the machine tool, tool, fixture, under the action of cutting force, the workpiece due to insufficient stiffness caused by deformation of the impact on the machining error is greater.


Tool stiffness: The stiffness of the external turning tool in the processing surface normal (y) direction is very large, its deformation is negligible. Boring a small diameter internal hole, the tool bar stiffness is very poor, the tool bar deformation by force has a great impact on the hole machining accuracy.


Stiffness of machine tool components: machine tool components are composed of many parts, machine tool component stiffness has so far no suitable simple calculation method, currently the main method is to determine the stiffness of machine tool components by experimental methods. Factors that affect the stiffness of machine tool components are the effect of contact deformation of the bond surface, the effect of friction, the effect of low stiffness parts, the effect of clearance.


6. Errors caused by thermal deformation of the process system


The effect of thermal deformation on machining errors is relatively large in process systems, especially in precision and large part machining, where machining errors caused by thermal deformation can sometimes account for up to 50% of the total workpiece error.


7. Adjustment error


In every process of machining, the process system has to be adjusted one way or another. Since adjustments cannot be absolutely accurate, they result in adjustment errors. In a process system, the accuracy of the mutual position of the workpiece and tool on the machine is ensured by adjusting the machine, tool, fixture or workpiece. Adjustment errors play a decisive role in machining errors when the original accuracy of machine tools, tools, fixtures, workpiece blanks, etc., is achieved without considering dynamic factors.


8. Measurement error


When a part is measured during or after machining, the measurement accuracy is directly affected by the measurement method, gage accuracy, and workpiece and subjective factors.


9. Internal stress


Stress that exists inside a part without external force is called internal stress. Once internal stress is generated on the workpiece, it will cause the workpiece metal to be in a high-energy unstable state, which instinctively transforms to a low-energy stable state, and with it deformation occurs, resulting in the loss of the original machining accuracy of the workpiece.

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